This grant application contains a descriptive proposal of a series of experiments designed to give direct information on the localization of calcium within the myocardium under various experimental situations. The proposed studies will utilize the well known isolated perfused heart preparation. Emphasis is placed upon the use of the inert nontoxic fluorocarbon liquids. They are used in colloidal suspensions and function as excellent oxygen carriers in the perfusion fluid. They are also used alone to physically fix the entire myocardium by freezing when they are perfused into the heart at minus 80 degrees C. Permanent fixation of ionic constituents and cellular architecture is affected by careful refrigerated drying. The primary question of this proposal will be resolved by isolating the various components of the desiccated myocardium and characterizing them according to both their morphology and their calcium content. Special emphasis is called to the technique whereby the various cellular structures are isolated; this being the use of isopycnic ultracentrifugation using continuous density gradients consisting of nonpolar liquids. Calcium content of these purified constituents will depend upon the use of the radioactive isotope Ca45, Morphological identification will be by means of specailized electron- microscopic techniques.